Re: [PATCH v2 2/5] mm: limit pagecache folio refcount when splicing into pipes

[David Hildenbrand (Arm)]

On 11/7/26 23:23, David Hildenbrand (Arm) wrote:
> Similar to mapping pagecache folios into page tables, let's reject
> additional references when splicing folios into a pipe if the refcount
> is at its limit, similar to how we handle it during GUP.
> 
> This will prevent more cases of possible folio refcount overflows with
> large pagecache folios.
> 
> Let's keep it simple and check in the two callers of
> splice_folio_into_pipe(). Return -EOVERFLOW if the limit prevents any
> progress, so do_splice_read() can fall back to copy_splice_read() when
> read_iter is available. This keeps sendfile() and other splice users
> working when ordinary reads would still succeed.
> 
> Also fall back to copying in FUSE when taking another reference through
> fuse_ref_folio() would exceed the limit.
> 
> Also check in try_get_page() to cover pipe-buffer duplication by tee()
> and partial pipe-to-pipe splicing.
> 
> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---
>  fs/fuse/dev.c      |  5 +++--
>  fs/splice.c        |  8 +++++++-
>  include/linux/mm.h |  9 ++++++---
>  mm/filemap.c       | 11 ++++++++---
>  mm/shmem.c         |  4 ++++
>  5 files changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 3641abb90d6bb..4f6b7b42bfd3e 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -1265,9 +1265,10 @@ int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop,
>  		if (cs->write && cs->pipebufs && folio) {
>  			/*
>  			 * Can't control lifetime of pipe buffers, so always
> -			 * copy user pages.
> +			 * copy user pages. Also copy folios at their refcount limit.
>  			 */
> -			if (cs->req->args->user_pages) {
> +			if (cs->req->args->user_pages ||
> +			    unlikely(folio_ref_count_over_limit(folio))) {

Linus points out off-list (thanks!), that fuse_ref_folio() should have
really used try_get_page() right from the start.

And that we should just not fallback to a copy, in particular,
if the refcount is invalid (<= 0).

So the following is what I have for now. Once we remove the limit
handling again, that code won't change, which is good.

I went back and forth with folio_get_checked() naming, but decided to
not mix in the "try" as that just reminds of our speculative refcount
handling. (folio_try_get_checked / folio_try_get_referenced)

I'll send a cleanup to rename try_get_page() later.

I played with using the new helper (folio_get_checked()) also in patch #1,
but it resulted in too much churn for something that is supposed to
be a temporary fix, so I'll leave that in place.